This invention relates to gas dynamic lasers and to the method of generating lasing using a gas flow.
Thermally pumped gas dynamic lasers utilize a gas flow to achieve the energy level population inversion necessary for lasing. A thermally pumped system begins with a hot equilibrium gas mixture in which there is no population inversion. The inversion is produced by rapid expansion of gas as it flows through a supersonic nozzle.
Both nitrogen and carbon dioxide are used in many gas dynamic lasers. The upper lasing energy level of the CO.sub.2 is very near the first vibrational energy level of nitrogen. In one form of a nitrogen-CO.sub.2 laser, an electrical arc plasma generator ionizes and heats the nitrogen. The gas is heated to about 2,000.degree. K. The nitrogen is at equilibrium as it enters a nozzle. The molecular nitrogen is expanded through the nozzle reaching supersonic velocity after leaving the nozzle throat. The supersonic expansion "freezes" the vibrational energy states of the thermally excited nitrogen. Cold CO.sub.2 is then injected into the gas flow downstream from the nozzle throat, where it mixes with the nitrogen (N.sub.2 to CO.sub.2 ratio of about 9:1) in the divergent exit portion of the nozzle. The molecules of nitrogen have been excited by the heating so that when the relatively cold CO.sub.2 is injected into the nitrogen stream, selected excitation of the upper lasing levels of the CO.sub.2 molecules occurs. The collision of the nitrogen and the CO.sub.2 transfers excitation energy. Thus a population inversion in the upper lasing levels of the CO.sub.2 is obtained. The excited CO.sub.2 exits the nozzle into a chamber where lasing occurs as the energy is released in the form of light.